1. Field of the Invention
The present invention relates to a watermark, and more particularly, to a method of designing a watermark having the power spectral density optimized so that the performance of detecting watermark can be improved by employing the whitening filtered detection after the Wiener attack.
2. Background of the Related Art
Generally, digital media such as audio, image and video have been rapidly increased. Since such digital media products can be easily distributed, forged and copied, it is necessary to protect their copyrights. Digital watermarking has been appeared as a popular means for satisfying such necessity. A digital watermark is hidden information permanently remaining in a digital signal without being perceived by humans. This watermark has information related to an owner, distributor and receiver of the digital signal. When the owner of this digital type work is inquired, it can be recognized through the extraction of inserted information.
As a security means for strengthening the copyright, the watermark should not be perceivable by the human visual system, the human auditory system and so on. Thus, the energy of the watermark should be small enough so that the watermark cannot be perceived. Also, the watermark should be robust so as to resist intentional or unintentional attack.
The unintentional attack means a loss of information in common signal processing such as lossy compression, low-pass filtering, scaling, digital-to-analog or analog-to-digital conversion, resampling, etc. The intentional attack means a removal operation of the watermark within preserving the quality of the digital contents.
Meanwhile, an algorithm for embedding the watermark should be secure enough so that the embedded watermark cannot be removed even if the algorithm becomes known. That is, only the copyright holder can access the watermark.
The most important thing is that the copyright holder can be accurately discriminated by the watermark.
A well-known power spectrum condition is derived as a defense against the Wiener attack. The Wiener attack means estimation and extraction of the embedded watermark from the signal including the watermark for the purpose of obstructing the detection of the watermark. According to the power spectrum condition, the power spectral density of the watermark should be in proportion to the power spectral density of the original signal. This is because it is difficult to estimate the watermark from the signal including the watermark in case of conforming to the power spectrum condition. However, the power spectrum condition considers only the detection based on a correlation, but does not consider employing the whitening filter prior to the detection.
FIG. 1 is a graph illustrating the power spectral density of the watermark and the original signal according to a conventional method.
According to the conventional method, the power spectral density of the watermark is designed to be in proportion to the power spectral density of the original signal. This proportion of the power spectral densities makes the estimation of the watermark due to an invasion and so on difficult. However, in the conventional watermark design, the whitening filter is not considered.
Meanwhile, in an article by Jonathan K. Su and Bernd Girod, entitled “On the Robustness and Imperceptibility of Digital Fingerprints”, Proc. IEEE Conf. on Multimedia Computing and Systems, pp. 530–535, vol. 2, June 1999, it is described that in most digital watermarking applications, the most preferable characteristics are robustness and imperceptibility. This article shows a theoretical analysis of the watermarking. Here, an attacker estimates the watermark using the Wiener filter.
The robustness and the imperceptibility of the watermark can be represented by an L2 efficiency, which indicates the energy of the watermark that the attacker can remove. The article shows that the power spectrum condition should be satisfied for the maximum L2 efficiency. That is, the power spectrum of the watermark should be the one obtained by reducing the power spectrum of the original signal in a specified ratio. A watermark that satisfies the power spectrum condition is the most efficient one among the watermarks independent from the original signal. In the analysis of one-dimensional and two-dimensional signal models and experiments using actual images, the watermark that satisfies the power spectrum condition shows an excellent performance.
Also, in U.S. Pat. No. 5,848,155, entitled “Spread Spectrum Watermark for Embedded Signaling”, a watermark is embedded into audio, video and image multimedia data using spread spectrum methodology. This patent overcomes the limitations of the prior systems using a spread spectrum watermarking method that embeds a watermark or certain information into perceptually significant regions of an image. Specifically, according to this patent, the watermark can be extracted from the watermarked data without using the original data. The preferred method of watermarking extraction is to use a spatial or temporal local average of the frequency coefficients of the watermarked data to determine the watermark. For instance, the frequency coefficients of a two-dimensional neighborhood in two-dimensional watermarked data, i.e., an image, are analyzed to reproduce the entire watermark. This is possible since the watermark is embedded into the data using spread spectrum technology that spreads the watermark throughout the data.
Meanwhile, the Wiener attack means to estimate the watermark using the Wiener filter and to obstruct the detection by subtracting the estimated watermark from the watermarked data. The robustness and the imperceptibility of the watermark can be measured by the energy of the estimated watermark that can be removed by the Wiener attack. Regarding the energy of the estimated watermark, the most efficient watermark should satisfy the power spectrum condition. That is, the power spectrum of the watermark is obtained by reducing the power spectrum of the original signal in a specified ratio. In case of using the matched filter only, the power spectrum condition compliant watermark shows the optimal performance against the Wiener attack. However, in case of employing the whitening filter prior to the detection, the whitening filter provides no gain for the power spectrum compliant watermark.